abstract: Raman scattering from Ge-Si core-shell nanowires is investigated theoretically and experimentally. A theoretical model that makes it possible to extract quantitative strain information from the measured Raman spectra is presented for the first time. Geometrical and elastic simplifications are introduced to keep the model analytical, which facilitates comparison with experimental results. In particular, the nanowires are assumed to be cylindrical, and their elastic constants isotropic. The simple analytical model is subsequently validated by performing numerical calculations using realistic nanowire geometries and cubic, anisotropic elastic constants. The comparison confirms that the analytic model is an excellent approximation that greatly facilitates quantitative Raman work, with expected errors in the strain determination that do not exceed 10%. Experimental Raman spectra of a variety of core-shell nanowires are presented, and the strain in the nanowires is assessed using the models described above. It is found that all structures present a significant degree of strain relaxation relative to ideal, fully strained Ge-Si core-shell structures. The analytical models are modified to quantify this strain relaxation. / Dissertation/Thesis / Ph.D. Physics 2011

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Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Gurpreet Singh / Molecular precursor derived ceramics (also known as polymer-derived ceramics or PDCs) are high temperature glasses that have been studied for applications involving operation at elevated temperatures. Prepared from controlled thermal degradation of liquid-phase organosilicon precursors, these ceramics offer remarkable engineering properties such as resistance to crystallization up to 1400 °C, semiconductor behavior at high temperatures and intense photoluminescence. These properties are a direct result of their covalent bonded amorphous network and free (-sp2) carbon along with mixed Si/B/C/N/O bonds, which otherwise can not be obtained through conventional ceramic processing techniques. This thesis demonstrates synthesis of a unique core/shell type nanowire structure involving either siliconboroncarbonitride (SiBCN) or siliconoxycarbide (SiOC) as the shell with carbon nanotube (CNT) acting as the core. This was made possible by liquid phase functionalization of CNT surfaces with respective polymeric precursor (e.g., home-made boron-modified polyureamethylvinylsilazane for SiBCN/CNT and commercially obtained polysiloxane for SiOC/CNT), followed by controlled pyrolysis in inert conditions. This unique architecture has several benefits such as high temperature oxidation resistance (provided by the ceramic shell), improved electrical conductivity and mechanical toughness (attributed to the CNT core) that allowed us to explore its use in energy conversion and storage devices. The first application involved use of SiBCN/CNT composite as a high temperature radiation absorbant material for laser thermal calorimeter. SiBCN/CNT spray coatings on copper substrate were exposed to high energy laser beams (continuous wave at 10.6 μm, 2.5 kW CO2 laser, 10 seconds) and resulting change in its microstructure was studied ex-situ. With the aid of multiple techniques we ascertained the thermal damage resistance to be 15 kW/cm2 with optical absorbance exceeding 97 %. This represents one order of magnitude improvement over bare CNTs (1.4 kW/cm2) coatings and two orders of magnitude over the conventional carbon paint (0.1 kW/cm2) currently in use. The second application involved use of SiBCN/CNT and SiOC/CNT composite coatings as energy storage (anode) material in a Li-ion rechargeable battery. Anode coatings (~1mg/cm2) prepared using SiBCN/CNT synthesized at 1100 °C exhibited high reversible (useable) capacity of 412 mAh/g even after 30 cycles. Further improvement in reversible capacity was obtained for SiOC/CNT coatings with 686 mAh/g at 40 cycles and approximately 99.6 % cyclic efficiency. Further, post cycling imaging of dissembled cells indicated good mechanical stability of these anodes and formation of a stable passivating layer necessary for long term cycling of the cell. This improved performance was collectively attributed to the amorphous ceramic shell that offered Li storage sites and the CNT core that provided the required mechanical strength against volume changes associated with repeated Li-cycling. This novel approach for synthesis of PDC nanocomposites and its application based testing offers a starting point to carry out further research with a variety of PDC chemistries at both fundamental and applied levels.

The monolithic integration of III-V semiconductors on Si substrates is a part of a long-term technological roadmap for the semiconductor industry towards More-than-Moore technologies. Despite of the different lattice constants and thermal expansion coefficients, research efforts over the last two decades have shown that III-V crystals with a high structural quality can be grown epitaxially in the form of nanowires directly on Si using CMOS-compatible (Au-free) methods. Among other III-V compounds, InxGa1-xAs is of the special interest for the use in infrared photonics and high-speed electronics due to its tunable direct bandgap and low electron effective mass, respectively. For comparison, InxGa1-xAs thin films are typically grown on lattice-matched InP substrates with a limited range of compositions at around x=0.52. The realization of InxGa1-xAs nanowires on Si, though, has been proved challenging owing to the limited In-content when the nanowires are grown Ga-catalyzed or the high density of stacking faults when the nanowires are grown catalyst-free. In this work, the use of highly lattice-mismatched GaAs/InxGa1-xAs and GaAs/InxAl1-xAs core/shell nanowires on Si(111) substrates have been studied as an alternative to InxGa1-xAs nanowires. The core/shell mismatch strain and its accommodation within the nanowires plays an important role in the growth, the structural, and the electronic properties of the nanowires. A key parameter in this work was the unusually small diameter of 20 – 25 nm of the GaAs core. First, the strain-induced bending of the nanowires during the growth of the shell by molecular beam epitaxy was investigated. It was apparent that the nanowires bend as a result of a preferential incorporation of In adatoms on one side of the nanowires. To obtain straight nanowires with symmetric shell composition and thickness around the core, it was necessary to choose relatively low growth temperatures and high growth rates that limited the surface diffusivity of In adatoms. Second, the strain accommodation in straight nanowires was investigated as a function of the shell thickness and composition using a combination of Raman scattering spectroscopy and X-ray diffraction. For a fixed shell composition of x=0.20 and small enough shell thicknesses, the strain in the shell is compressive and decreases progressively as the shell grows thicker. On the other hand, the strain in the core is tensile with hydrostatic character and increases with shell thickness. Finally, for shell thicknesses larger than 40 nm, the shell becomes strain-free, whereas the strain in the core saturates at 3.2% without any dislocations. For a fixed shell thickness of 80 nm, the strain in the core was further increased by increasing the In-content in the shell, reaching values as high as 7% for x=0.54. A plastic relaxation via misfit dislocations was observed only for the next highest In-content of x=0.70. In agreement to theoretical predictions, the tensile strain in the core resulted in a large reduction of the GaAs bandgap (as measured by photoluminescence spectroscopy), up to approximately 40% of the strain-free value. A similar reduction in electron effective mass is also expected. The transport properties of electrons inside the strained GaAs core were assessed by optical-pump terahertz-probe spectroscopy. Quite high mobility values of approximately 6100 cm2/Vs at 300 K for a carrier concentration of 9×1017 cm−3 were measured, which are the highest reported in the literature for GaAs nanowires, but also higher than the values for unstrained bulk GaAs. The importance of the results in this work is two-fold. On the one hand, strain-free InxGa1-xAs nanowire shells were grown on Si substrates with x up to 0.54 and thicknesses well beyond the critical thickness of their thin film counterparts. Such shells could potentially be employed as conduction channels in high electron mobility transistors (HEMTs) integrated in Si platforms. On the other hand, highly tensile-strained GaAs cores with electronic properties like those of InxGa1-xAs thin films were obtained. In this case, the results demonstrate, that GaAs nanowires can be suitable for photonic devices across the near-infrared range, including telecom photonics at 1.3 and potentially 1.55 μm, as well as for high-speed electronics. GaAs as a binary material is expected to be advantageous compared to InxGa1-xAs due to the absence of structural imperfections typically present in ternary alloys. Finally, to explore the potential of the core/shell nanowires as HEMTs, self-consistent Schrödinger-Poisson calculations of two different modulation-doped heterostructures were performed. In the case of a strained GaAs core overgrown by an unstrained InxGa1-xAs shell and an additional unstrained Si-doped InxAl1-xAs shell, the possibility to form a cylindrical-like two-dimensional electron gas inside the InxGa1-xAs shell was found. In the alternative case of a strained GaAs core overgrown by an unstrained Si-doped InxAl1-xAs shell, it was found that it is possible to form a quasi-one-dimensional electron gas at the center of the core. Both structures are the subject of ongoing research.:1 Introduction 1 2 Fundamentals and state-of-the-art 7 2.1 Electronic and structural properties of III-V semiconductors 7 2.2 Growth of III-V nanowires on Si 20 2.3 Core/shell heterostructure nanowires 29 2.4 Strain in epilayers and core/shell nanowires 36 2.5 Strain engineering in core/shell nanowires and its effect on band parameters 46 2.6 Modulation-doped III-V semiconductor heterostructures 56 3 Methods 61 3.1 Optical and electron microscopes 61 3.2 X-ray diffraction 64 3.3 Raman scattering spectroscopy 65 3.4 Photoluminescence spectroscopy 75 3.5 Optical-pump terahertz-probe spectroscopy and photoconductivity in semiconductors 77 3.6 Device processing 82 3.7 Semiconductor nanodevice software “nextnano” 85 3.8 MBE for crystal growth and core/shell nanowire growth 86 4 Results and discussions 91 4.1 Structural, compositional analyses of straight nanowires and coherent growth limit 91 4.2 Bent nanowires 95 4.3 Strain analyses in core/shell nanowires 97 4.3.1 Dependence of strain on shell thickness 97 4.3.2 Dependence of strain on the shell chemical composition 102 4.3.3 Dependence of strain on the core diameter 105 4.4 Strain-induced modification of electronic properties 106 4.5 Strain-enhanced electron mobility of GaAs nanowires higher than the bulk limit 114 4.6 Towards high electron mobility transistors 123 5 Conclusion and outlook 129 Bibliography 131 List of abbreviations I List of Symbols III List of publications VII List of conference contributions VIII Acknowledgements X

Abstract Template assisted deposition was used to produce various nanomaterials including simple nanowires, nanorods, multi-segmented metal nanowires, core-shell nanowires, alloy and polymer wires and tubes. Anodized aluminum oxide (AAO) membranes were used as templates for the growth of the various structures using an electrochemical deposition method and also by wetting the porous templates. In the electrochemical deposition method, the pore size of the templates affects the rate of synthesis and the structures of the nanomaterials while in the wetting method, the viscosity and reaction time in the polymer solution influence the structures of the nanomaterials. A conventional two-step anodization procedure was used to synthesize thick AAO templates with porous hexagonal channels at a constant applied voltage and temperature. A maximum thickness of over 180 µm oxide layer could be fabricated using mild anodization at 60 V and 80 V. Compared to conventional mild anodization, these conditions facilitated faster growth of oxide layers with regular pore arrangement. Polyethylene glycol (PEG) containing ferromagnetic nanowires were synthesized using template assisted electrochemical deposition method. During the synthesis, simultaneous deposition of polymer and metal ions resulted nanowires coated with a uniform layer of PEG without interfering with the structure and magnetic properties of the nanowires. PEG-coated Ni nanowires were embedded in polyethylene diacrylate (PEGDA) matrix after the removal of the AAO templates. Comparison of results with and without a magnetic field during embedding showed that the presence of magnetic field supported embedding of nanowire arrays in polymer. Influence of using AAO templates with several pore diameters for the synthesis of bimetallic nanowires were studied by growing Ni-Fe and Ni-Co bi-metallic nanowires. At a constant applied current by using templates with a pore diameters of 60 nm alloy formed while with a pore diameter of 130 nm core-shell nanowires formed. Polyvinylidene fluoride (PVDF) films and nanotubes were synthesized using a solution recrystallization method that favored the formation of piezoelectric β phase thin films. Variation in the concentration of polymer in the mixture solution allowed synthesis of different types of structures such as PVDF composites, nanorods and nanocrystals with tunable morphologies. Keywords: One-dimensional structures, electrodeposition, porous alumina, ferromagnetic nanostructures, magnetic core-shell nanowires, alloys, polymer composite, stimuli-active, PEGDA, azobenzene, and PVDF.

This thesis is devoted to the investigation of mechanical properties and self-assembly process of materials at the nanoscale. Various nanostructured materials such as nanoparticles, nanotubes, nanowires and thin films are used as constituent elements of nanodevices. Hence, knowledge of the mechanical properties of materials at the nanoscale is extremely important for understanding their functionality in nanodevices. Mechanical properties of nanostructured materials may significantly differ from those of their bulk counterparts due to the high surface to volume ratio in nanostruc-tures. We particularly focus on the role of the surface region on the stiffness of nanomaterials. We have shown that the stiffness of a nanomaterial can be tuned over a wide range by introducing appropriate coating on the nanostructure surface. We have also explored the effects of the surface region on the stability of various phases in a nanostructure. In the second part of this thesis, we have described the self-assembly process of nanostructures mediated by drendrimers. Self-assembly techniques are frequently used to decorate nanostructures into specific networks. The motivation of this study is to investigate the mechanisms which control the effective interaction and the inter-particle distance between nanoparticle-dendrimer compos-ites. Control over the inter-particle separation is very important since it has a strong influence on the electronic and optical properties of the nanostructures. In the following paragraphs, we sum-marize the results of our study. We start with a brief introduction to the mechanical properties and self-assembly process of nanostructures in the first chapter. A brief review of the work done on these topics in the recent past is presented in this chapter. We discuss the results and conclusions of various experimental and numerical studies on these topics. We also mention the motivation for the studies we have carried out. At the end, we briefly describe the numerical methods (molecular dynamics (MD) and density functional theory (DFT)) which have been used in our investigations. In the second chapter, we discuss the effects of the surface region on the mechanical properties of nanostructures. We have investigated the size and growth direction dependence of the mechanical properties of ZnS nanowires and thin films as a case study. We observe that the Young’s modulus of nanowires and thin films strongly depends on their size and growth direction. This size and growth direction dependence of the stiffness of nanostructured materials can be explained in terms of their surface modifications. Since the energy of the surface region is usually higher than that of the core region in a nanostructure, the surface atoms move their positions to minimize the surface energy. As a result, bond lengths at the surface region are usually different from their bulk values. We observe that in ZnS nanowires and thin films, the average bond length at the surface region is lower than that in the core region which remains unchanged from its bulk value. This decrease in the bond length (or equivalently increase in the bond energy) increases the effective stiffness of the entire nanostructure. As the size of the nanowire/thin film increases, the effect of the surface region gradually decreases and hence the Young’s modulus value converges to the bulk value. Since the surface region has a strong influence on the mechanical properties of nanostructures, the stiffness of a nanostructure can be tuned by modifying the surface region with other materials. In chapter three, we have shown that the stiffness of ZnS nanowires can be tuned by introducing a thin CdS shell on top of the ZnS surface. In general, the stiffness of a nanostructure can be increased (decreased) by coating the surface region with a stiffer (less stiff) material. However, the stiffness of the core/shell nanostructures strongly depends on the properties of the interface between the core and the shell. We observe that the binding energy between the core and shell regions is relatively low due to the lattice mismatch at the interface region of core/shell nanostructures. This lower binding energy strongly affects the stiffness of core/shell nanostructures. We have also shown that thermal properties such as thermal conductivity and melting temperature of core/shell structures can be tuned by changing the coating material. In chapter four, we discuss the effects of the surface region on the stability of various phases in a nanostructure. The surface atoms may stabilize a particular phase in a nanostructure which is not a stable phase in the bulk material. In this chapter, we investigate the stability of the h-MgO phase, an intermediate structure found during the wurtzite to rock salt transformation, in CdSe nanostructures. We observe that this five-fold coordinated phase is more stable at lower temperatures and smaller sizes of the nanowires. The appearance of this phase has not been observed till now in experiments. We show that this phase is not stable for larger CdSe nanocrystals on which the experiments have been done. In the rest of the thesis, we have presented the results of our studies of self-assembly of nanostructures mediated by DNAs and dendrimers. First we describe in chapter five the nature of the effective interaction between two PAMAM dendrimers. Dendrimers are frequently used to coat surfaces of nanoparticles to prevent the nanoparticles from aggregation. The interaction between such nanoparticle-dendrimer composites depends strongly on the nature of the effective interac-tion between dendrimers. We have used fully atomistic MD simulations to calculate the potential of mean force (PMF) between two PAMAM dendrimers. We show that the effective interaction strongly depends on the size (generation) and protonation level of the dendrimers. The PMF profiles of nonprotonated dendrimers show a global minimum which represents the attractive nature of the interaction between the dendrimers up to a certain center-to-center distance. On the other hand, the interaction between protonated dendrimers is repulsive throughout their interaction re-gion. The PMF profiles are fitted very well by a sum of an exponential and a Gaussian function. This observation is in contradiction with some of the results of existing coarse-grained simulations which predicted the effective interaction between dendrimers to be Gaussian. Our atomistic simulation which includes all the local fluctuations is expected to give more accurate results. Information about the effective interaction between two dendrimers helps in understanding how dendrimer molecules can be used to control the interaction strength and the preferred inter-particle distance between two nanostructures. In chapter six, we discuss the effective interaction between two dendrimer grafted gold nanoparticles. We observe that dendrimer molecules can get adsorbed spontaneously on the surface of a gold nanoparticle. These grafted dendrimers significantly alter the interaction between the gold nanoparticles. We have explored the effects of proto-nation level and the density of the grafted dendrimers on the effective interaction between two gold nanoparticle-dendrimer composites. We observe that these nanoparticle-dendrimer composites at-tract each other at low grafting density. However, the interaction strength and the inter-particle distance at the minimum of the potential are much lower and higher, respectively than those between two bare gold nanoparticles. Interestingly at higher grafting density, the nature of the interaction between the nanocomposites depends on the protonation level of the grafted dendrimers. Nanoparticles grafted with nonprotonated dendrimers still attract each other but with lower inter-action strength and higher inter-particle distance compared to the values for low grafting density. On the other hand, nanocomposites grafted with protonated dendrimers repel each other at high grafting density. Thus we show that the effective interaction and the optimal inter-particle distance between the nanostructures can be tuned over a wide range by using a suitable grafting density and protonation level of the dendrimers. In the seventh chapter, we describe a strategy to assemble dendrimers with the help of sin-gle stranded DNA (ssDNA). We attach an ssDNA to one dendrimer and a complementary ssDNA to a second dendrimer. These two complementary ssDNAs bind with each other through base pair formation to assemble the dendrimers into a single structure. The complementary ssDNAs form a dsDNA which is rigid enough to maintain the inter-dendrimer distance almost the same as the length of the DNA. The inter-dendrimer distance can be tuned by changing the DNA length. However, this method strongly depends on the protonation level of the dendrimers. It works well only for nonprotonated dendrimers. Since the protonated dendrimers are positively charged, they strongly interact with the negatively charged ssDNAs through electrostatic interaction. As a result, ssDNAs wrap the dendrimer surface and hence the inter-dendrimer distance can not be controlled. We have also verified that this method works for multiple nonprotonated dendrimers as well. In the final chapter of this thesis, we summarize the main results and conclude with a brief discussion of future directions of research on the problems considered in the thesis.

Le travail illustré par ce manuscrit de thèse présente l'étude structurale d'hétéro-structures semi-conductrices à base de nitrures d'éléments III avec l'un des outils les plus puissants de la recherche scientifique: le rayonnement synchrotron. La cartographie haute résolution de l'espace réciproque, la diffraction anomale multi-longueur d'onde, la spectroscopie d'absorption X et la spectroscopie en condition de diffraction nous ont permis de caractériser la structure à l'échelle atomique de différentes régions d'un même système. Dans un premier temps, nous montrons que les nanofils GaN sur Si(111) ont une polarité N et proposons un mécanisme de nucléation. Dans un second temps, nous mettons en évidence un phénomène d'inter-diffusion stimulée par la contrainte dans les boîtes quantiques GaN / AlN recuites à haute température. Enfin, nous observons la présence d'un ordre local à courte distance dans les nanofils coeur-coquille InGaN / GaN. Cette organisation atomique pourrait être induite par la présence de contrainte, nous avons initié une étude de l'anisotropie de l'ordre à courte distance pour explorer cette hypothèse. / The work presented in this manuscript deals with the structural investigation of III-nitrides semiconductor heterostructures with one of the most powerful tools of materials science: the synchrotron radiation. We used high resolution reciprocal space mapping, multi-wavelenght anomalous diffraction, x-rays absorption spectroscopy and diffraction anomalous fine structure to characterize the structure at the atomic scale of complex nano-structured systems. First, we show that GaN nanowires on Si(111) are N-polar and we suggest a nucleation mecanism. Then, we highlight a strain assited inter-diffusion phenomenon in the GaN / AlN quantum dots annealed at high temperature. Finally, we observe the presence of short-range order in InGaN / GaN core-shell nanowires. This atomic organization could be induced by the presence of stress and we have begun a study of the anisotropic ordering to explore this hypothesis.